Apparatus and methods consistent with the present invention relate to a plasma arc torch and, more particularly, to a novel electrode for use in a plasma arc torch having an improved service life and a method of making the same.
Commonly used for working of metals, plasma arc torches are used for cutting, welding, surface treatment, melting and annealing. These torches include an electrode that supports an arc that extends from the electrode to the workpiece in the transferred arc mode of operation. It is also conventional to surround the arc with a swirling vortex of gas, and in some torch designs, it is conventional to envelope the gas and arc with a swirling jet of water.
The electrode used in a conventional torch of the type described typically comprises an elongate tubular member having a material of high thermal conductivity, such as copper or copper alloy. The forward or discharge end of the tubular electrode includes a bottom end wall having an emissive insert embedded therein, which supports the arc. The insert includes a material which has a relatively low work function, which is defined in the art as the potential step, measured in electron volts, which permits thermionic emission from the surface of a metal at a given temperature. In view of its low work function, the insert is thus capable of readily emitting electrons when an electrical potential is applied thereto, and commonly used insert materials include hafnium, zirconium, and tungsten.
One of the major problems connected with the torches referred to above is the shortness of service life of their electrodes, especially when the torches are used with an oxidizing arc gas, such as oxygen or air. In those torches, the gas appears to rapidly oxidize the copper, and as the copper oxidizes, its work function fails. As a result, the oxidized copper that surrounds the insert begins to support the arc in preference to the insert. After this occurs, the copper melts, thereby causing early destruction and/or failure of the electrode.
U.S. Pat. No. 5,023,425 (Severance, Jr.) which issued on Jun. 11, 1991, and which is incorporated herein by reference, discloses an electrode for a plasma arc torch wherein the electrode includes a copper holder having a lower end which mounts an emissive insert that acts as the cathode terminal for the arc during operation. A sleeve of silver is positioned in a cavity to surround the insert and forms an annular ring on the lower end surface of the holder to surround the exposed end face of the emissive insert. The annular ring serves to prevent arcing from the copper holder, and maintains the arc on the insert. However, while the silver sleeve of the ""425 patent was intended to prolong the life of the copper holder, in practice, this electrode suffers from problems in that the wear does not come from double arcing, but from the hafnium overheating and eroding.
U.S. Pat. No. 3,930,139 (Bykhovsky et al.) which issued on Dec. 30, 1975, and which is incorporated herein by reference, also discloses an electrode for plasma arc working of materials. In the ""139 patent, the holder is again formed from copper or copper alloys and an active insert is fastened to the end face of the holder and is in thermal and electrical contact with the holder through a metal distance piece disposed between the active insert and the holder and over the entire contact surface area. The metal distance piece is formed from aluminum or aluminum alloys and the active insert is formed from hafnium or from hafnium with yttrium and neodymium oxides as dopants therein taken separately or in combination. However, while the aluminum sleeve surrounding the active insert in the ""139 patent serves to protect the copper holder surrounding the active insert, the aluminum distance piece or sleeve offers no advantages over the silver sleeve of the ""425 patent to Severance, Jr.
U.S. Pat. No. 5,676,864 (Walters) which issued on Oct. 14, 1997, discloses an electrode for a plasma arc torch wherein the electrode includes a copper holder having a lower end which mounts an emissive insert that acts as the cathode terminal for the arc during operation. A sleeve of silver is positioned substantially to surround the insert and form an annular ring on the lower end surface of the holder to surround the exposed end face of the emissive insert. The insert assembly further includes an aluminum face plate disposed in the enlarged outer portion of the cavity and which is exposed at the front end of the metallic holder so as to surround a front portion of the sleeve.
U.S. Pat. No. 5,767,478 (Walters) which issued on Jun. 16, 1998, alternatively teaches eliminating the aluminum face plate of U.S. Pat. No. 5,676,864 and instead provides for the front end of the holder to directly contact the emissive insert forming an overlay portion of the holder between the front face thereof and the sleeve, thus protecting the silver sleeve. However, the use of a silver sleeve as described above does not provide for a good mechanical bond with the surrounding copper holder, and could be improved.
It is an intention of the present invention to provide an electrode adapted for use in a plasma arc torch of the type described, and which is capable of providing significantly improved service life when the torch is used in an oxidizing atmosphere.
In particular, the present invention provides an electrode for supporting an arc in a plasma arc torch. The electrode includes a metallic holder including a front end casting portion having an emissive insert cavity and an outer face; and a rear end portion joined to the front end portion. An emissive insert is mounted in the emissive insert cavity and is a metallic material having a relatively low work function, such as hafnium, zirconium, tungsten, and alloys thereof. The front end casting portion is a metal which is selected from silver, gold, platinum, rhodium, indium, palladium, nickel and alloys thereof. The rear end portion is a metal selected from copper and copper alloys. The emissive insert is a metal selected from hafnium, zirconium, tungsten, and alloys thereof.
It is also contemplated that an embodiment of the present invention includes a cylindrical post which extends inwardly from the outer face of the front end portion. The cylindrical post has an inner-rear tip portion made of a metal selected from copper and copper alloys. It is also further contemplated that the rear end portion may be a casting. It is even further contemplated that an embodiment of the present invention includes casting means for bonding together a front end portion, a rear end portion, and an emissive insert.
The invention also includes a method of fabricating an electrode adapted for supporting an arc in a plasma arc torch. The electrode comprises a holder having a front end portion and a rear end portion. The method includes the machining of a piece of material to form a rear end portion blank and the forming of a notch in one end of the rear end portion blank, so that an end of an emissive insert can be inserted into the notch. A metal selected from silver, gold, platinum, rhodium, indium, palladium, nickel and alloys thereof, is melted and used to form the front end portion of the holder by being cast onto the emissive insert and onto an end of the rear end portion blank, resulting in the formation of a front end portion blank. The front end portion blank and the rear end portion blank constitute a holder blank. The holder blank is then machined to form a finished holder.
The method further contemplates the machining of the holder to form an externally threaded portion and an internal cavity. The internal cavity extends through the rear end portion and into the front end portion of the holder. The material for the rear end portion blank is selected from copper and copper alloys.
The method still further contemplates that the forming of the internal cavity results in the formation of a cylindrical post which extends inwardly from the front end portion of the holder. The cylindrical post has an inner-rear tip portion, which includes a part of the rear end portion, which remained after the cavity was formed.
Features of the electrode of the present invention include adaptability for use in a plasma arc torch of the type described. The invention also provides a significant advantage from the perspective of improved service life when the torch is used in an oxidizing atmosphere. More specifically, the silver (and other suitable metals as described above) gives good conductivity and provides a cooler flow of electricity to the emissive insert and better heat flow out of the emissive insert and through the silver. The joining of the three materials by casting creates a bond that allows a less resistant flow of electrons through each material. The lack of resistance allows the emissive insert to maintain a cooler temperature, and last much longer.